An original reading device (hereunder referred to as a large-scale image sensor) comprises an insulating substrate, a plurality of photoelectric transducers formed on the substrate, and a circuit for switching and scanning the transducers. The circuit is either formed on the substrate or attached to another substrate. The sensor uses an optical fiber array or lens array to read the original, so the length of the optical path for image forming can be decreased to reduce the scale of the reading device by a considerable degree.
The driving circuit of the conventional large-scale image sensor and its construction are shown in FIGS. 1(a) and 1(b), and a cross-section of FIG. 1(b) taken on the line A--A' is shown in FIG. 1(c). A light receiving element generally indicated at 5 comprises a substrate 1 supporting a thin photoconductive film 3 sandwiched between discrete electrodes 2, made of a thin conductive film, and a continuous electrode 4 made of a thin transparent conductive film. An equivalent circuit of the light receiving element consists of a photodiode PD and a capacitor CD in parallel.
A shift register 8 turns on MOS transistors 7.sub.1 to 7.sub.n sequentially, and a bias supply 10 charges light receiving elements 5.sub.1 to 5.sub.n to some maximum voltage. In between charging, the voltage across the capacitor CD reduces depending on the quantity of light falling on the element 5, i.e. The capacitor CD is discharged by the photodiode PD whose conductivity depends upon the light intensity. During the next cycle, when shift register 8 turns on the transistors 7.sub.1 -7.sub.n, the capacitors CD are charged back to the maximum voltage. The amount of charge required to fully charge each capacitor CD is a measure of the light intensity on the photodiodes between charging cycles. The charging signal is picked up on line 12 as a video signal by a load resistor 9 and constitutes an output signal of the array. In short, a recharging current flows in the area where discharging has occurred due to illumination, and no recharging current flows in the black area where no discharge has taken place. Each resolvable spot is known as a pixel, which is short for picture element.
A light receiving element having a pixel density of, say, 8 dots/mm and a length equal to the size of the original (210 mm) is formed on the insulating substrate 1. The MOS transistors 7.sub.1 to 7.sub.n and the shift register 8 are packed in an integrated switching circuit 6 which is mounted on the substrate 1 and connected to the element 5 by wire bonding 11 or other suitable means.
The conventional arrangement described above requires a very long signal line 12 which causes the following disadvantages. First, noise is induced in the signal line 12 to reduce the S/N ratio considerably. Typical noises are clocked noise accompanying the driving of the shift register 8 and spike noise that enters from the gates of the MOS transistors 7.sub.1 to 7.sub.n. Secondly, increased parasitic capacitance 13 on the signal line 12 increases the CR time constant in the reading circuit and decreases the sharpness of the waveform of the charging current, thereby decreasing the reading speed. The parasitic capacitance 13 is the sum of the stray capacitance on the signal line 12 and the source-ground capacitance of the MOS transistors 7.sub.1 to 7.sub.n and it increases with the increase in the length of the signal line 12 and the number of MOS transistors connected to the line. Because of these disadvantages, the requirements of high-speed reading and low power consumption have not been met by the conventional large-scale image sensor.